Lithium secondary battery and method of manufacturing the same

ABSTRACT

The present invention uses a magnesium-based alloy which is excellent in mechanical workability to be formed thinner than conventional alloys. The present invention provides a lithium secondary battery comprising an electrode assembly and a non-aqueous electrolyte, both accommodated in a metal jacket, wherein the metal jacket is made of a magnesium-based alloy containing lithium in an amount of 7 to 20% by weight; and a metal layer or an insulating layer for preventing corrosion of the metal jacket is formed integrally with the metal jacket on the inner wall thereof.

TECHNICAL FIELD

The present invention relates to a lithium secondary battery and amethod of manufacturing the same. More specifically, the inventionrelates to a metal jacket of a lithium secondary battery made of amagnesium-based alloy containing lithium (Mg—Li alloy).

BACKGROUND ART

With the recent prevalence of portable apparatuses, demands forsecondary batteries have been increased. In particular, a lithiumsecondary battery containing an organic electrolyte, which enables areduction in the size and weight of such a portable apparatus, hasobtained a rapidly increasing share in the market. Though the majorityof conventional lithium secondary batteries have cylindrical orcoin-like shapes, the number of secondary batteries having rectangularshapes have begun increasing recently. In addition, sheet-like thinbatteries have made their debut.

It is very important to increase the energy density of a battery. Theenergy density of a battery can be expressed by volume energy density(Wh/liter), which indicates the size of a battery, and weight energydensity (Wh/kg), which indicates the weight of a battery. From theviewpoint of a reduction in size and weight, a battery is required tohave a higher volume energy density and weight energy density, because akeen competition exists in the market of such batteries.

The energy density of a battery is determined mainly by active materialsof the positive and negative electrodes as the power generatingelements. Other important determinants include the electrolyte and theseparator. Improvements in these determinants are being intensively madein pursuit of a battery having a higher energy density.

A metal jacket for accommodating such power generating elements is alsoreviewed as an important factor of a reduction in size and weight of abattery and is being improved actively. If the metal jacket has athinner wall, larger quantities of the active materials can beaccommodated within the metal jacket in a conventional shape. This leadsto an improvement in the volume energy density of the battery.Alternatively, if the weight of the metal jacket can be reduced, theweight of the battery in a conventional shape can be reduced. This leadsto an improvement in the weight energy density of the battery.

One known example of a battery with a light metal jacket is a lithiumion battery in a rectangular shape that employs a metal jacket made of alight aluminum-based alloy sheet (specific gravity: about 2.8 g/cc)instead of a conventional steel sheet (specific gravity: about 7.9g/cc). In the technical field of batteries for use in cellular phones,there is known a case where the weight energy density of a battery hasbeen increased by about 10% by employing the metal jacket made of analuminum-based alloy (refer to JAPANESE PATENT LAID-OPEN GAZETTE No. HEI8-329908).

Many methods of manufacturing such metal jackets made of aluminum oraluminum-based alloy have an impact process or a drawing process.

Attention has recently been focused on magnesium-based alloys, which arelighter than aluminum or aluminum-based alloys. The specific gravity ofmagnesium is 1.74 g/cc, whereas that of aluminum is 2.7 g/cc. Examplesof well-known magnesium-based alloys include alloys comprising magnesiumadmixed with Al, Zn or the like. Some cases are known where amagnesium-based alloy is used for the metal jacket of a battery (referto JAPANESE PATENT LAID-OPEN GAZETTE Nos. HEI 11-25933 and HEI11-86805).

Further, attention has recently been paid to magnesium-based alloyscontaining lithium which have superplasticity (refer to JAPANESE PATENTLAID-OPEN GAZETTE No. HEI 6-65668). Magnesium-based alloys containinglithium are characterized in that they have smaller specific gravity(about 1.3 to 1.4 g/cc) than pure Mg and are superior in mechanicalworkability to conventional magnesium-based alloys containing Al.

There is, however, not known any case where a magnesium-based alloycontaining lithium is applied to a metal jacket for a battery.

Meanwhile, thixomolding process is receiving attention as a noveltechnique for processing magnesium-based alloys in the art of structuralmaterial for use in various electric appliances. Thixomolding process isa modification of the diecasting process, which has been the mainstreamof the conventional technology, and is similar to the injection moldingprocess for plastics. Specifically, this process injects a raw materialalloy in a semi-molten state into a mold, solidifies the raw materialalloy, and then removes the molded product from the mold. The resultantcrystal of the molded product is not of a dendritic structure, whichresults from the diecasting process, but of a granular structure, whichresults from the solidification process under stress. The alloy having agranular structure exhibits such features as improved mechanicalproperties and stabilized quality even when made thin.

There is, however, not known any case where a magnesium-based alloycontaining lithium obtained by thixomolding process is applied to ametal jacket for a battery.

Though there are some cases where magnesium-based alloys are used formetal jackets (JAPANESE PATENT LAID-OPEN GAZETTE Nos. HEI 11-25933 andHEI 11-86805) as described above, the conventional magnesium-basedalloys have poor workability and hence have been difficult to be appliedto metal jackets for batteries practically. Further, sincemagnesium-based alloys are corroded when brought into contact with apower generating element such as an electrolyte, the use of amagnesium-based alloy has not been practical in view of realizing asatisfactory charge/discharge cycle.

DISCLOSURE OF INVENTION

The present invention has been made to provide a lithium secondarybattery having a higher capacity and a lighter weight than the prior artbattery. To this end, the present invention uses, as the raw material ofa metal jacket, a specified magnesium-based alloy containing lithium(Mg—Li alloy) that can be subjected to a processing of bending, deepdrawing or the like in the cold work, the process having been considereddifficult for conventional magnesium-based alloys. In one embodiment ofthe present invention, a light and high-strength lithium secondarybattery of high quality is manufactured by the use of a sheet of amagnesium-based alloy containing lithium obtained by the thixomoldingprocess.

In the present invention, the metal jacket is prevented from corrosiondue to contact with the electrolyte or the like by forming a metal layeror an insulating layer integrally with the metal jacket on the innerwall thereof. As a result, it becomes possible to realize a stabilizedcharge/discharge cycle, which has been considered impossible to realizefor a battery having a metal jacket made of a magnesium-based alloy.

Among light batteries, in the case of a battery using a metal jacketmade of an aluminum or aluminum-based alloy, the negative electrode ofthe battery cannot be connected to the metal jacket. This is because theconnection between the metal jacket and the negative electrode wouldfacilitate the production of an intermetallic compound such as AlLi thatmakes the metal jacket brittle. The majority of conventional batteries,however, have a structure in which the metal jacket is connected to thenegative electrode. From the viewpoint of obtaining a general-purposebattery, it is desired that the metal jacket should be electricallyconnected to the negative electrode.

The metal jacket of the battery in accordance with the presentinvention, in contrast, is free from an inconvenience such asembrittlement even when electrically connected to the negative electrodeby virtue of the metal layer or the insulting layer formed integrallywith the metal jacket. Thus, the battery of the present invention issuperior in terms of versatility also.

The present invention is directed to a lithium secondary batterycomprising an electrode assembly and a non-aqueous electrolyte, bothaccommodated in a metal jacket, wherein the metal jacket is made of amagnesium-based alloy containing lithium in an amount of 7 to 20% byweight; and a metal layer is formed integrally with the metal jacket onthe inner wall thereof for preventing corrosion of the metal jacket. Theelectrode assembly comprises a positive electrode, a negative electrodeand a separator in general.

In this battery, the magnesium-based alloy containing lithium preferablycontains lithium in an amount of 7 to 15% by weight, and at least oneelement selected from the group consisting of Al, Zn, Mn, Zr, Ca, Si,and rare earth elements in a total amount of 0.3 to 5% by weight.

Alternatively, the magnesium-based alloy containing lithium may be abinary alloy containing lithium in an amount of 12 to 16% by weight.

The metal layer for preventing corrosion of the metal jacket preferablycomprises Ni or Cu.

Further, the metal layer is preferably formed by cladding, plating orvapor-deposition.

The present invention is also directed to a lithium secondary batterycomprising an electrode assembly and a non-aqueous electrolyte, bothaccommodated in a metal jacket, wherein the metal jacket is made of amagnesium-based alloy containing lithium in an amount of 7 to 15% byweight, and at least one element selected from the group consisting ofAl, Zn, and Mn in a total amount of 0.3 to 5% by weight; an Ni layerhaving a thickness of 2 to 20 μm is formed integrally with the metaljacket on the inner wall thereof by cladding; and the metal jacket iselectrically connected to a negative electrode in the electrodeassembly.

In this construction, the magnesium-based alloy containing lithium ispreferably produced by thixomolding.

Preferably, the metal jacket is in a shape of a bottomed can with anopen top, having a bottom/side wall thickness ratio (bottom wallthickness/side wall thickness) of 1.1 to 2.0, and the magnesium-basedalloy containing lithium is produced by thixomolding.

The present invention is also directed to a method of manufacturing alithium secondary battery, comprising the steps of: (1) preparing asheet of a magnesium-based alloy containing lithium in an amount of 7 to15% by weight, and at least one element selected from the groupconsisting of Al, Zn, and Mn in a total amount of 0.3 to 5% by weight bythixomolding; (2) forming a Ni layer integrally with the sheet on atleast one face thereof by cladding; (3) forming a metal jacket in ashape of a bottomed can with an open top with the Ni layer formed on theinner wall thereof from the sheet by a mechanical processing selectedfrom drawing, combined drawing and ironing, and impact; and (4) placingan electrode assembly and a non-aqueous electrolyte into the metaljacket.

The present invention is yet also directed to a lithium secondarybattery comprising an electrode assembly and a non-aqueous electrolyte,both accommodated in a metal jacket, wherein the metal jacket is made ofa magnesium-based alloy containing lithium in an amount of 7 to 20% byweight; and an insulating layer is formed integrally with the metaljacket on the inner wall thereof.

In this construction, the magnesium-based alloy containing lithiumpreferably contains lithium in an amount of 7 to 15% by weight, and atleast one element selected from the group consisting of Al, Zn, Mn, Zr,Ca, Si, and rare earth elements in a total amount of 0.3 to 5% byweight.

Alternatively, the magnesium-based alloy containing lithium may be abinary alloy containing lithium in an amount of 12 to 16% by weight.

The insulating layer preferably comprises a metal oxide or a resin.

The present invention is still also directed to a lithium secondarybattery comprising an electrode assembly and a non-aqueous electrolyte,both accommodated in a metal jacket, wherein the metal jacket is made ofa magnesium-based alloy containing lithium in an amount of 7 to 15% byweight, and at least one element selected from the group consisting ofAl, Zn, and Mn in a total amount of 0.3 to 5% by weight; and a resinlayer having a thickness of 5 μm or more is formed integrally with themetal jacket on the inner wall thereof.

In this construction, the magnesium-based alloy containing lithium ispreferably produced by thixomolding.

Preferably, the metal jacket is in a shape of a bottomed can with anopen top, having a bottom/side wall thickness ratio (bottom wallthickness/side wall thickness) of 1.1 to 2.0, and the magnesium-basedalloy is produced by thixomolding.

The present invention is still yet directed to a method of manufacturinga lithium secondary battery, comprising the steps of: (1) preparing asheet of a magnesium-based alloy containing lithium in an amount of 7 to15% by weight, and at least one element selected from the groupconsisting of Al, Zn, and Mn in a total amount of 0.3 to 5% by weight bythixomolding; (2) forming a resin layer integrally with the sheet on atleast one face thereof; (3) forming a metal jacket in a shape of abottomed can with an open top with the resin layer formed on the innerwall thereof from the sheet by a mechanical processing selected fromdrawing, combined drawing and ironing, and impact; and (4) placing anelectrode assembly and a non-aqueous electrolyte into the metal jacket.

The present invention is further directed to a method of manufacturing alithium secondary battery, comprising the steps of: (1) preparing asheet of a magnesium-based alloy containing lithium in an amount of 7 to15% by weight, and at least one element selected from the groupconsisting of Al, Zn, and Mn in a total amount of 0.3 to 5% by weight bythixomolding; (2) forming a metal jacket in a shape of a bottomed canwith an open top from the sheet by a mechanical processing selected fromdrawing, combined drawing and ironing, and impact; (3) forming a resinlayer integrally with the metal jacket on the inner wall thereof; and(4) placing an electrode assembly and a non-aqueous electrolyte into themetal jacket.

While the novel feature of the invention are set forth particularly inthe appended claims, the invention, both as to organization and content,will be better understood and appreciated, along with other objects andfeatures thereof, from the following detailed description taken inconjunction with the drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view illustrating a cup made of an alloy before beingintroduced into an orifice formed by dies to conduct a combined drawingand ironing process for obtaining a metal jacket.

FIG. 2 is a view illustrating a cup made of an alloy passing through anorifice at the final die in a combined drawing and ironing process forobtaining a metal jacket.

FIG. 3 is a vertical sectional view showing an examplary metal jacket ofa lithium secondary battery in accordance with present invention.

FIG. 4 is a vertical sectional view showing the structure of anexemplary lithium secondary battery in accordance with the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

The metal jacket for use in the present invention is made of amagnesium-based alloy containing lithium in an amount of 7 to 20% byweight. The magnesium-based alloy has superplasticity. When the contentof lithium in the alloy is less than 7% by weight, the mechanicalworkability of the alloy lowers. When the content of lithium in thealloy is more than 20% by weight, on the other hand, the corrosionresistance of the alloy becomes insufficient.

In the case that the magnesium-based alloy is a binary alloysubstantially composed of only Mg and Li, the content of Li in the alloyis preferably 12 to 16% by weight. In the case that the magnesium-basedalloy comprises three or more elements, on the other hand, the contentof Li in the alloy is preferably 7 to 15% by weight. Such multi-elementalloys comprising three or more elements preferably have a lower contentof Li than the binary alloy.

Herein, any one of the alloys for use in the present invention maycontain unavoidable impurities.

Preferably, the multi-element alloy contains element X, which is atleast one element selected from the group consisting of Al, Zn, Mn, Zr,Ca, Si, and rare earth elements, in an amount of 0.3 to 5% by weightbesides lithium in an amount of 7 to 15% by weight. The rare earthelements herein include lanthanoid elements (⁵⁷La to ⁷¹Lu), Sc, and Y.

By appropriately selecting one or more element from the aforementionedelements as the element X and the content thereof within theaforementioned range, it is possible to obtain an alloy having desiredmechanical workability and corrosion resistance.

For the purpose of improving the strength of the alloy, use of Al as theelement X, in particular, is preferable. For the purpose of improvingthe mechanical properties of the alloy, use of Zn as the element X ispreferable. For the purpose of improving the corrosion resistance of thealloy, use of Mn as the element X is preferable.

When the content of the element X in the alloy is less than 0.3% byweight, the effect of the addition of the element X becomesinsufficient. When the content of the element X in the alloy is morethan 5% by weight, on the other hand, the resultant alloy tends toexhibit lowered mechanical workability.

Examples of specific alloys suitable for the metal jacket for use in thepresent invention include:

alloy (a): a binary alloy comprising 84 to 88% by weight of Mg, and 12to 16% by weight of Li;

alloy (b): a ternary alloy comprising 80 to 92.7% by weight of Mg, 7 to15% by weight of Li, and 0.3 to 5.0% by weight of Al;

alloy (c): a ternary alloy comprising 80 to 92.7% by weight of Mg, 7 to15% by weight of Li, and 0.3 to 5.0% by weight of Zn;

alloy (d): a ternary alloy comprising 80 to 92.7% by weight of Mg, 7 to15% by weight of Li, and 0.3 to 5.0% by weight of Mn;

alloy (e): a ternary alloy comprising 80 to 92.7% by weight of Mg, 7 to15% by weight of Li, and 0.3 to 5.0% by weight of Zr;

alloy (f): a multi-element alloy comprising 80 to 92.7% by weight of Mg,7 to 15% by weight of Li, and 0.3 to 5.0% by weight of misch metal (Mm:a mixture of rare earth elements);

alloy (g): a ternary alloy comprising 80 to 92.7% by weight of Mg, 7 to15% by weight of Li, and 0.3 to 5.0% by weight of Y;

alloy (h): a quaternary alloy comprising 80 to 92.7% by weight of Mg, 7to 15% by weight of Li, and a total amount of 0.3 to 5.0% by weight ofAl and Zn;

alloy (i): a quaternary alloy comprising 80 to 92.7% by weight of Mg, 7to 15% by weight of Li, and a total amount of 0.3 to 5.0% by weight ofAl and Mn; and

alloy (j): a multi-element alloy comprising 80 to 92.7% by weight of Mg,7 to 15% by weight of Li, and a total amount of 0.3 to 5.0% by weight ofAl and Mm.

Though the method for obtaining a metal jacket is not limited to anyspecified method, a method employing a mechanical processing of a sheetof the alloy into a shape of a bottomed can is generally conducted.Preferable mechanical processing include drawing, combined drawing andironing, and impact.

Generally, the magnesium-based alloy containing lithium is formed into ametal jacket in a shape of a bottomed can with an open top, such as acylindrical or rectangular shape, or analogous shape thereto with anopen top.

With reference to FIGS. 1 and 2, an example of a combined drawing andironing process is described. Here, the case where a bottomedcylindrical metal jacket with an open top is formed by this process isdescribed.

First, a bottomed cylindrical cup 11 is formed from an alloy sheet.FIGS. 1 and 2 illustrate a process of forming the bottomed cylindricalcup 11 into a desired shape, with relevant portions in section.

In FIGS. 1 and 2, four ironing dies 12 a to 12 d are coaxially disposedin tiers. As shown in FIG. 1, the cup 11 is set on the upper portion ofthe ironing die 12 a and then is forced into the central orifice formedby the dies by means of a punch 13. The diameter of the orifice islarger at a die through which the cup 11 passes earlier than at thesucceeding die. The diameter of the punch 13 corresponds to the innerdiameter of a desired metal jacket, while the diameter of the orifice atthe last die 12 d corresponds to the outer diameter of the desired metaljacket. Accordingly, the inner and outer diameters of the cup 11 becomesmaller consecutively as the cup 11 passes through the orifice, therebyobtained a desired metal jacket. FIG. 2 shows cup 11′ passing throughthe orifice at the last die 12 d, which has reduced inner and outerdiameters and is elongated vertically. When the punch 13 is drawn out ofthe cup 11′, a desired metal jacket is obtained. The side wall of thecup 11′ is made thinner than that of the cup 11 by ironing. However, thethickness of the bottom wall does not vary significantly.

The side wall of a metal jacket obtained by the combined drawing andironing or the like is thinner than the bottom wall. An alloy of higherworkability tends to provide a higher bottom/side wall thickness ratio(bottom wall thickness/side wall thickness). A higher bottom/side wallthickness ratio is more effective in downsizing and weight-reduction ofa battery. A preferable bottom/side wall thickness ratio ranges 1.1 to2.0. When the bottom/side wall thickness ratio is less than 1.1, anunsatisfactory weight-reduction effect is likely. When the ratio is morethan 2.0, the mechanical strength and the reliability of the battery arelikely to lower.

In terms of further improvements in workability and mechanical strength,and more stabilized quality of the battery, the magnesium-based alloycontaining lithium is preferably prepared by thixomolding. Specifically,the alloy is preferably obtained by injection-molding of a semi-moltenalloy having a large thixotropy.

Two methods can be chiefly used for forming the metal jacket from analloy prepared by thixomolding. One is to inject the semi-molten alloydirectly into a mold having the inner shape corresponding to the shapeof the metal jacket. The other is to prepare an alloy sheet by usinginjection-molding of the semi-molten alloy previously and then conduct amechanical processing of the resultant alloy sheet for forming a metaljacket as described above. In terms of productivity, the latter method,i.e., a method employing a mechanical processing, is generally employed.

In using the magnesium-based alloy containing lithium for the metaljacket of a lithium secondary battery, it is necessary to take thecorrosion resistance of the alloy and the structure of the battery intoconsideration. For this reason, the metal jacket used in the presentinvention is formed integrally with a metal layer for preventingcorrosion of the metal jacket on the inner wall thereof. Alternatively,the metal jacket is formed integrally with an insulating layer on theinner wall thereof.

An example of such a metal jacket having a metal layer 11 c is shown inFIG. 3 schematically, wherein the thickness of the metal layer 11 c isnot accurately represented. Reference numerals 11 a and 11 b denote thebottom wall and the side wall, respectively, of the metal jacket. Asdescribed above, the side wall thickness (TB) is smaller than the bottomwall thickness (TA).

The metal jacket formed integrally with such a metal layer on the innerwall thereof is described first.

The magnesium-based alloy containing lithium is easy to corrode whenbrought into contact with an power generating element such as anelectrolyte. Therefore, the metal layer formed on the inner wall of themetal jacket is required to comprise a metal that is stable against thepower generating elements of the battery. In this respect, the metallayer is preferably a layer comprising Ni or Cu, for example. Inobtaining the general-purpose battery, it is preferred that the metaljacket having a Ni layer on the inner wall thereof should be connectedto the negative electrode.

Preferably, the metal layer is formed by a cladding process, wherein twoor more thin metal sheets are piled and joined to each other integrally.Specifically, a metal sheet of Ni, Cu or the like is superposed on asheet of the magnesium-based alloy to form a clad plate, which in turnis formed into the metal jacket having the metal layer on the inner wallthereof. Alternatively, such a metal layer may be formed on a sheet ofthe magnesium-based alloy by a chemical process such as plating or aphysical process such as vapor-deposition. Instead, a metal paste may beapplied to a sheet of the magnesium-based alloy. Alternatively, it ispossible that the metal jacket is formed in the manner described aboveand then the metal layer is formed on the inner wall of the metal jacketby plating or vapor-deposition.

The thickness of the metal layer is preferably 2 to 20 μm. When thethickness of the metal layer is less than 2 μm, the effect preventingcorrosion of the metal jacket is likely to become insufficient. On theother hand, when its thickness is more than 20 μm, thecorrosion-preventive effect is saturated, whereas the effect reducingthe weight of the battery is impaired. To ensure an adequatecorrosion-preventive effect, the metal layer is preferably made to havea thickness of 5 to 20 μm.

In turn, the metal jacket formed integrally with the insulating layer onthe inner wall thereof is described below.

The insulating layer is required to comprise a material that is stableagainst any power generating element of the battery. In this respect,the insulating layer is preferably a layer comprising a metal oxide or aresin, for example. From the viewpoint of easy handling, use of a resinlayer is particularly preferable.

The formation of the metal oxide layer is conveniently achieved by, forexample, a process of positively oxidizing the inner wall of the metaljacket. The formation of the resin layer is preferably achieved by aprocess including spraying a resin dispersion onto the inner wall of themetal jacket and heating the sprayed resin dispersion to a temperatureequal to or higher than the melting point of the resin component of thedispersion after drying. With this process, the resin component ismelted by heating and is turned into a film which is firmly integratedwith the inner wall of the metal jacket.

Polyethylene, polypropylene or the like is suitable as the resin formingthe resin layer in terms of their excellent corrosion resistance to theelectrolyte.

Preferably, the thickness of the insulating layer is 5 μm or more. Whenits thickness is less than 5 μm, the corrosion-preventive effect islikely to become insufficient. When its thickness is more than 200 μm,for example, the battery is unlikely to have a higher energy density asdesired.

Some examples of preferred methods of manufacturing the lithiumsecondary battery in accordance with the present invention are listedbelow.

Embodiment 1

First, a sheet of a magnesium-based alloy containing lithium in anamount of 7 to 15% by weight, and at least one element selected from thegroup consisting of Al, Zn, and Mn in a total amount of 0.3 to 5.0% byweight is prepared.

Next, the prepared sheet and a Ni sheet are piled and joined to eachother to form a clad plate having a Ni layer on at least one facethereof.

Subsequently, the clad plate is subjected to a mechanical processingselected from drawing, combined drawing and ironing, and impact to forma metal jacket in a shape of a bottomed can having the Ni layer on theinner wall thereof.

Finally, an electrode assembly and a non-aqueous electrolyte are placedinto the metal jacket.

Embodiment 2

First, a sheet of a magnesium-based alloy containing lithium in anamount of 7 to 15% by weight, and at least one element selected from thegroup consisting of Al, Zn, and Mn in a total amount of 0.3 to 5.0% byweight is prepared.

Next, the prepared sheet is subjected to a mechanical processingselected from drawing, combined drawing and ironing, and impact to forma metal jacket in a shape of bottomed can.

Subsequently, a resin dispersion containing a resin component such aspolyethylene, polypropylene or the like is sprayed onto the inner wallof the metal jacket, followed by drying, and then the sprayed resincomponent is heated to a temperature equal to or higher than the meltingpoint of the resin component.

Finally, an electrode assembly and a non-aqueous electrolyte are placedinto the metal jacket.

An example of the structure of a cylindrical lithium secondary batteryis described below with reference to FIG. 4 showing the structure partlyin section.

In FIG. 4, a battery case 1 is a metal jacket having a Ni layer on theinner wall thereof. Since the Ni layer is very thin relative to thethickness of the metal jacket, the depiction of the Ni layer is omittedfrom FIG. 4.

Within the battery case 1 are accommodated power generating elementsincluding an electrode assembly 4 and a non-aqueous electrolyte, whereasthe non-aqueous electrolyte is not depicted in FIG. 4. The electrodeassembly 4 comprises a positive electrode plate 5, a negative electrodeplate 6, and a separator 7, wherein the positive electrode plate 5 andthe negative electrode plate 6 are laid upon another with the separator7 inserted therebetween and wound.

Insulating rings 8 are disposed above and below the electrode assembly 4to prevent short-circuiting. A positive electrode lead 5 a connected tothe positive electrode plate 5 is extended through the upper insulatingring 8 and is electrically connected to a sealing plate 2 serving as apositive terminal. On the other hand, a negative electrode lead 6 aconnected to the negative electrode 6 is extended through the lowerinsulating ring and is electrically connected to the battery case 1serving as a negative terminal. The clearance between the openingportion of the battery case 1 and the sealing plate 2 is sealed with aninsulating packing 3.

Hereinafter, the present invention will be concretely described by wayof examples.

EXAMPLE 1

A metal jacket was formed using a magnesium-based ternary alloycontaining lithium, and a cylindrical lithium secondary battery A wasmanufactured using the metal jacket.

In the formation of the metal jacket, a ternary alloy comprising 84.8%by weight of Mg, 14% by weight of Li, and 1.2% by weight of Al was used.This alloy was subjected to thixomolding to give a thin sheet having athickness of 0.5 mm. Next, a 20 μm-thick nickel foil was superposed onone face of the thin sheet, followed by rolling to provide a clad plate.This clad plate was punched into a disk, and the resultant disk wassubjected to a combined drawing and ironing process to give acylindrical metal jacket in a shape of a bottomed can with an open tophaving an outer diameter of 13.8 mm and a height of 54.0 mm. Thisprocess was performed so that the nickel foil side should form the innersurface of the resultant metal jacket. The open end portion of the metaljacket was cut off.

As a result, there was obtained a cylindrical metal jacket having anouter diameter of 13.8 mm, a height of 49.0 mm, a bottom wall thicknessof 0.5 mm, a side wall thickness of 0.4 mm, and a bottom/side wallthickness ratio of 1.25. The side wall thickness was measured at amidpoint in the vertical height and was considered to be an averagethickness of the side wall. The metal jacket thus obtained had aremarkably reduced weight as small as about ½ of that of a conventionalmetal jacket made of an aluminum-based alloy.

Next, positive and negative electrodes and a separator as powergenerating elements were prepared as following.

The positive electrode used was obtained by applying a paste comprisingLiCoO₂, Acetylene black and a fluorocarbon polymer onto an aluminumfoil, drying, rolling and cutting to a predetermined size.

The negative electrode used was obtained by applying a paste comprisingspherical graphite, styrene-butadiene rubber, carboxymethylcellulose andwater onto a copper foil, drying, rolling and cutting to a predeterminedsize.

The separator used was a microporous polyethylene film having athickness of 0.027 mm.

The electrolyte used was prepared by mixing ethylene carbonate anddiethyl carbonate in a molar ratio of 1:3 and dissolving lithiumhexafluorophosphate (LiPF₆) into the mixture to a concentration of 1mol/liter.

The cylindrical lithium secondary battery was assembled with use of themetal jacket as follows.

First, a stack of the positive electrode and the negative electrode withthe separator intervening between the two was wound to give an electrodeassembly so that the negative electrode form the outermost layer of theassembly. The electrode assembly thus formed was placed into theaforementioned metal jacket. This resulted in direct electricalconnection made between the metal jacket and the negative electrode.Subsequently, the electrolyte was injected into the metal jacket. Thepositive electrode, on the other hand, was connected to a positiveelectrode lead of aluminum, which was connected to a sealing plateserving as a positive terminal. The open top of the metal jacket wasthen sealed with the sealing plate. At this time, an insulating packingwas placed between the sealing plate and the peripheral portion of themetal jacket.

The battery A thus obtained was an AA-size cylindrical battery having adiameter of 14 mm, a height of 50 mm, and a battery capacity of 600 mAh.

COMPARATIVE EXAMPLE 1

A conventional aluminum-based alloy was used to form a metal jackethaving the same shape as and equal bottom wall thickness and side wallthickness to the battery A. With use of this metal jacket, a lithiumsecondary battery B was assembled in the same manner as in Example 1.The aluminum-based alloy used in the battery B was an Al 3003 alloycontaining manganese.

Since the aluminum-based alloy was used for the metal jacket in thebattery B, the relation between the positive electrode and the negativeelectrode in the battery B was reverse to that in the battery A.Accordingly, the metal jacket was electrically connected directly to thepositive electrode plate. The capacity of this battery B was 600 mAh.

The batteries A and B are different from each other in the raw materialof metal jacket. The metal jacket of the battery A is lighter than thatof the battery B and hence has an advantage in terms of weight energydensity of the battery.

Further, the battery A is superior to the battery B in mechanicalstrength. It is, therefore, possible to further make the metal jacket ofthe battery A thinner, though the metal jackets of example 1 andcomparative example 1 were both equally made to have a side wallthickness of 0.4 mm and a capacity of 600 mAh.

Because of the difference in the raw material of the metal jacket, thebattery A was lighter by about 0.5 g than the battery B despite the factthat the capacity of the battery A was equal to that of the battery B.

Next, the charge/discharge cycle life of each battery was evaluated.

Specifically, the batteries A and B were respectively subjected to aconstant-voltage and constant-current charging at an electric current of0.6 A or less at 20° C. up to a voltage of 4.2 V and then subjected to aconstant-current discharging at a current of 120 mA at 20° C. down to afinal voltage of 3 V. This charge/discharge cycle was repeated 500times. This cycle life test proved that both of the batteries A and Bexhibited very stable performance up to the 500th cycle. Thus, thebatteries A and B were found to have substantially equalcharge/discharge cycle life. In the evaluation of other batteryproperties, there was not observed any significant difference betweenthe two batteries.

Thus, it was proved that the use of the metal jacket made of amagnesium-based alloy containing lithium, which had conventionally beenconsidered to have a poor corrosion resistance, made it possible toprovide a battery having both higher energy density and higherreliability.

EXAMPLES 2-29 COMPARATIVE EXAMPLES 2-7

The composition of the alloy to be used for the metal jacket of acylindrical lithium secondary battery was studied.

In this study were used Mg—Li (x) binary alloys shown in Table 1,Mg—Li(x)-X¹(y) ternary alloys shown in Table 2, and Mg—Li(x)-X¹(y)-X²(z)quaternary alloy shown in Table 3. In these tables, X¹ and X² representAl, Zn, Mn, Zr, Mm or Y, independently, and x, y and z represent thecontent in wt. % of Li, X¹ and X² in the alloy, respectively, thebalance of the alloy being Mg. It should be noted that this studyemployed the same conditions as in example 1 except the composition ofthe alloy used.

First, metal jackets were formed using the alloys of the respectivecompositions shown in Tables 1 to 3 in the same manner as in example 1.Among them, metal jackets that could be formed without any problem wereused to manufacture a battery in the same manner as in example 1.Batteries thus obtained were evaluated as in example 1.

TABLE 1 Composition of alloy used for metal jacket (x in parenthesesrepresents the content of Li in wt % Batteries and the balance is Mg)Comparative C1 Mg—Li(x) x = 5  Example 2 Example 2 C2 x = 10 Example 3C3 x = 15 Example 4 C4 x = 20 Comparative C5 x = 25 Example 3Comparative C6 x = 30 Example 4

TABLE 2 Composition of alloy used for metal jacket (x and y inparentheses represents the content of Li, and Al, Zn, Mn, Zr, Mm or Y,Batterie respectively, in wt % and the balance is Mg) Comparative D1 Mg—Li(x)—Al(y) x = 5  y = 1 Example 5 Example 5 D2  x = 10 y = 1 Example6 D3  x = 15 y = 1 Example 7 D4  x = 20 y = 1 Comparative D5  x = 25 y =1 Example 6 Comparative D6  x = 30 y = 1 Example 7 Example 8 D7  x = 10  y = 0.1 Example 9 D8  x = 10   y = 0.3 Example 10 D9  x = 10 y = 2Example 11 D10 x = 10 y = 4 Example 12 D11 x = 10 y = 6 Example 13 D12Mg—Li(x)—Zn(y) x = 10   y = 0.1 Example 14 D13 x = 10   y = 0.3 Example15 D14 x = 10 y = 1 Example 16 D15 x = 10 y = 2 Example 17 D16 x = 10 y= 4 Example 18 D17 x = 10 y = 6 Example 19 D18 Mg—Li(x)—Mn(y) x = 10 y =1 Example 20 D19 Mg—Li(x)—Zr(y) x = 10 y = 1 Example 21 D20Mg—Li(x)—Mm(y) x = 10 y = 1 Example 22 D21 Mg—Li(x)—Y(y) x = 10 y = 1

TABLE 3 Composition of alloy used for metal jacket (x, y and z inparentheses represents the Bat- content of Li, and Al or Zn, and Zn, Mnor Mm, teries respectively, in wt % and the balance is Mg) Example E1Mg—Li(x)—Al(y)—Zn(z) x = 10 y = z = 23 0.02 0.01 Example E2 x = 10 y = z= 24 2 1 Example E3 x = 10 y = z = 25 2 2 Example E4 x = 10 y = z = 26 41 Example E5 x = 10 y = z = 27 4 3 Example E6 Mg—Li(x)—Al(y)—Mn(z) x =10 y = z = 28 2 1 Example E7 Mg—Li(x)—Zn(y)—Mm(z) x = 10 y = z = 29 2 1

In the manufacture of metal jackets, the Mg—Li (x=5) alloy(corresponding to battery C1) had a drawback in mechanical processing.The Mg—Li (x=10) alloy (corresponding to battery C2) exhibited somewhatimproved workability as compared with the Mg—Li (x=5) alloy, which,however, was still insufficient. Any one of the Mg—Li (x=15 or more)alloys (corresponding to batteries C3 to C6) exhibited good workability.

Among all the ternary and quaternary alloys, only the Mg—Li(x=5)-Al(y=1)ternary alloy (corresponding to battery D1) had a drawback inworkability.

All the batteries shown in tables 1 to 3 except batteries C1, C2 and D1were assembled and then evaluated in the same manner as in example 1.

As a result, any one of the batteries except batteries C5, C6, D5, D6,D7, D11, D12, D17 and E5 was found to realize charge/discharge cycleswithout any problem and hence to exhibit a long charge/discharge cyclelife.

On the other hand, any one of the batteries C5, C6, D5, D6, D7, D11,D12, D17 and E5 exhibited a somewhat decreased electric capacity by the500th cycle. When each of these batteries was disassembled, the metaljacket was found to have corroded.

In view of the results thus obtained, the composition of each alloy wasstudied in more detail. As a result, the following considerations can begiven.

When roughly sorted, there are two major factors determining acomposition of the alloy to be used for a metal jacket. One is themechanical workability, and the other is the corrosion resistance.Optimal compositions satisfying these two requirements are thosecontaining magnesium as a major component, and lithium in an amount of 7to 20% by weight. Among them, the magnesium-based binary alloypreferably contains lithium in an amount of 12 to 16% by weight. Themagnesium-based multi-element alloy comprising three or more elementspreferably contains lithium in an amount of 7 to 15% by weight, and theelement X in an amount of 0.3 to 5% by weight, wherein the element X isat least one element selected from the group consisting of Al, Zn, Mn,Zr, Ca, Si, and rare earth metallic elements. It should be noted thatthough only the results of the binary to quaternary alloys are shown inthe foregoing tables, a battery E8 using a Mg—Li(x)-Al(y)-Zn(z)-Mn(v)quinary alloy wherein x=10, y=2, z=0.5, and v=0.5, for example, wasconfirmed to exhibit performance equivalent to that of the battery E6 orE7.

The following knowledge was obtained from the study.

The addition of Li to Mg can decrease the density of the resultingalloy. The addition of Li is known to cause the crystal structure tochange from α-phase of hexagonal close-packed structure to β-phase ofbody-centered cubic structure with increasing amount of Li added. Thisβ-phase provides a great leap in improving the workability of the alloyin cold work. A practically optimal amount of Li to be added is about 10to 15% by weight in the resulting alloy.

The effect of the addition of the third component X added tomagnesium-based alloy containing lithium is briefly described below.

Al, for example, has effects of improving the strength and corrosionresistance but decreasing the ductility, malleability and impactresistance of the alloy. Zn improves the mechanical properties, and Mnimproves the corrosion resistance of the alloy. Si forms anintermetallic compound (Mg₂Si) thereby improving the creep properties ofthe alloy. A rare earth metallic element contributes to an improvementin strength as well as corrosion resistance of the alloy.

Next, with respect to magnesium-based alloy containing lithium, atypical casting process and thixomolding process were studied bycomparison with each other.

A ternary alloy containing 84.8% by weight of Mg, 14% by weight of Li,and 1.2% by weight of Al was obtained by the typical casting processusing a high frequency induction furnace, and the alloy thus obtainedwas mechanically rolled to give a sheet having a thickness of 0.5 mm. Onthe other hand, the thixomolding process was used to give an alloy sheetof the same composition having a thickness of 0.5 mm.

For determining the limit of the thickness of the metal jacket, thesesheets were subjected to combined drawing and ironing process as inexample 1 to form respective metal jackets. In this case, the limit ofthe ratio of bottom wall thickness (TA)/side wall thickness (TB) wasdetermined. As a result, the limit of the ratio of TA/TB wassubstantially 1.5 for the sheet obtained by the casting process. Thesheet obtained by the thixomolding process, on the other hand, couldattain a TA/TB ratio of about 2.5 to 3.0 without any problem and henceexhibited higher workability. The limit was judged by the occurrence ofa fracture, crack or the like. It was proved from the foregoing that analloy prepared by thixomolding process was capable of providing a metaljacket having a thinner wall than an alloy having the same compositionbut prepared by the typical casting process. The metal jacket having aTA/TB ratio of about 1.1 to 2.0 can enjoy a sufficient weight-reductioneffect.

EXAMPLES 30-32

Sorts of metal layers to be applied on the inner wall of the metaljacket for preventing corrosion of the metal jacket and the optimalthickness of the metal layer were studied using cylindrical lithiumsecondary batteries.

The alloy used for the metal jacket was the same alloy as used inexample 1. Three sorts of metals, i.e., Ni, Cu and Al, were selected asthe metal to be applied onto the inner wall of the metal jacket. Inintegrating the metal layer with the the metal jacket by cladding, thethickness of the resulting metal layer after the manufacture of themetal jacket was adjusted to about 10 μm.

First, suitableness of each metal was examined.

As a result, batteries F1 (corresponding to example 30) and F2(corresponding to example 31) having a Ni layer and a Cu layer,respectively, were found to exhibit satisfactory properties includingstabilized charge/discharge cycle property. By contrast, a battery F3(corresponding to example 32) having an Al layer on the inner wallthereof was found to have unsatisfactory charge/discharge cycleproperty. The battery F3 did not exhibit satisfactory performancebecause, presumably, aluminum formed a compound with lithium and becamebrittle, and lithium reacted with aluminum became stabilized and henceinactive for the discharge reaction of the battery.

It is, however, known that the case where Al is used for the metal layercan provides a battery exhibiting satisfactory cycle life if the metaljacket is connected to the positive electrode (namely the metal jacketis used as the positive terminal), in lieu of the connection to thenegative electrode as in the foregoing cases.

EXAMPLES 33-42

Selecting Ni layer as the metal layer based on the results obtainedabove, research was conducted to optimize the thickness of the metallayer.

Metal jackets were formed so that the Ni layers on the inner wallsthereof should have average thicknesses of 0.5, 1, 2, 5, 10, 15, 20, 25,30 and 50 μm, respectively. In the same manner as in example 1,batteries were assembled and then subjected to the charge/dischargecycle life test. The batteries having the Ni layers of 0.5, 1, 2, 5, 10,15, 20, 25, 30 and 50 μm thicknesses are herein termed batteries G1, G2,G3, G4, G5, G6, G7, G8, G9 and G10, respectively, which correspond toexamples 33, 34, 35, 36, 37, 38, 39, 40, 41 and 42, respectively.

As a result, the batteries having the 0.5 μm-thick Ni layer and the 1μm-thick Ni layer, respectively, exhibited relatively low cycle lifeproperty, where the thinner the thickness, the lower the cycle lifeproperty. On the other hand, the batteries with their Ni layer havingthickness of 2 μm or more were stable in terms of the cycle life andfree from any problem. Accordingly, the lower limit of the Ni layerthickness is probably 2 μm. For reliably preventing the occurrence of apinhole or the like, the Ni layer thickness is preferably 5 μm or more.Though the possibility of corrosion decreases as the thicknessincreases, the upper limit of the Ni layer thickness is preferably about20 μm since too thick Ni layer will increase the weight of the wholemetal jacket significantly.

Although Ni layer was selected as the metal layer and integrated withthe metal jacket by cladding in the above test, Cu layer is alsoexcellent as the metal layer, and the metal layer can effectively formedby plating, vapor-deposition or the like instead of cladding.

EXAMPLE 43

Sorts of insulating layers to be applied on the inner wall of a metaljacket and the optimal thickness of the layer were studied usingcylindrical lithium secondary batteries.

A resin was selected as the insulating material. In this case, a metaljacket was formed previously in the same manner as in example 1.

Fine powder of polyethylene was mixed with an aqueous solution ofcarboxymethylcellulose to prepare a viscous slurry, which in turn wasapplied onto the inner wall of the metal jacket. The slurry thus appliedwas dried by heating at about 130° C., with the result that the innerwall of the metal jacket was covered with an insulating layer comprisingpolyethylene having a thickness of about 10 μm.

This insulating layer was uniform, had a sufficient mechanical strength,and was firmly integrated with the metal jacket.

In the same manner as in example 1, a lithium secondary battery H wasassembled using the metal jacket thus formed. In this case, since theinner wall of the metal jacket was electrically insulated from thenegative electrode by the insulating layer formed thereon, a lead wasused to electrically connect the negative electrode and the metaljacket.

In the same manner as in example 1, the performance of the battery H wasevaluated by testing the charge/discharge cycle life thereof. As aresult, there was not observed any problem up to the 500th cycle, and,hence the battery H was confirmed to have a long cycle life.

Study of other resins revealed that polypropylene exhibited an effect asexcellent as polyethylene. The thickness of the insulating layer ispreferably 5 μm or more from the viewpoint of the cycle life property.Though the energy density of the battery lowers to some extent withincreasing thickness of the insulating layer, such an insulating layerprovides, by virtue of its flexibility, an additional effect ofadvantageously relieving strain from the swelling of theelectrode-active material which occurs as the charge/discharge cycleproceeds. Thus, even a considerably thick insulating resin layerprovides advantages in terms of cycle life.

Like the foregoing resins, metal oxides are also effective as thematerial of the insulating layer. A preferred process for forming ametal oxide layer comprises directly oxidizing a surface of themagnesium-based alloy sheet. Further, a combination of a metal oxide anda resin to form the insulating layer is effective for improving theadherence of the insulating layer to the metal jacket.

While examples of the present invention have been described by way ofthe cylindrical lithium secondary batteries, the present invention isalso effectively applicable to rectangular lithium secondary batteries.In the manufacture of a rectangular battery, a sheet of an alloy to beformed into a metal jacket is desirably punched ovally.

INDUSTRIAL APPLICABILITY

As has been described, the present invention makes it possible toprovide a highly reliable and safe lithium secondary battery whichallows a reduction in weight and an increase in energy density byachieving improvements in the mechanical workability of the alloy to beformed into a metal jacket and the corrosion resistance of the metaljacket at one time, which have conventionally been left unachieved, byuse of a magnesium-based alloy which is capable of making the metaljacket lighter and thinner. Although the present invention has beendescribed in terms of the presently preferred embodiments, it is to beunderstood that such disclosure is not to be interpreted as limiting.Various alterations and modifications will no doubt become apparent tothose skilled in the art to which the present invention pertains, afterhaving read the above disclosure. Accordingly, it is intended that theappended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

What is claimed is:
 1. A lithium secondary battery comprising anelectrode assembly and a non-aqueous electrolyte, both accommodated in ametal jacket, wherein said metal jacket is made of a magnesium-basedalloy containing lithium in an amount of 7 to 20% by weight; and a metallayer for preventing corrosion of said metal jacket is formed integrallywith said metal jacket on an inner wall thereof.
 2. The lithiumsecondary battery in accordance with claim 1, wherein saidmagnesium-based alloy contains lithium in an amount of 7 to 15% byweight, and at least one element selected from the group consisting ofAl, Zn, Mn, Zr, Ca, Si, and rare earth elements in a total amount of 0.3to 5% by weight.
 3. The lithium secondary battery in accordance withclaim 1, wherein said magnesium-based alloy is a binary alloy containinglithium in an amount of 12 to 16% by weight.
 4. The lithium secondarybattery in accordance with claim 1, wherein said metal layer comprisesNi or Cu.
 5. The lithium secondary battery in accordance with claim 1,wherein said metal layer is formed by cladding, plating orvapor-deposition.
 6. A lithium secondary battery comprising an electrodeassembly and a non-aqueous electrolyte, both accommodated in a metaljacket, wherein said metal jacket is made of a magnesium-based alloycontaining lithium in an amount of 7 to 15% by weight, and at least oneelement selected from the group consisting of Al, Zn, and Mn in a totalamount of 0.3 to 5% by weight; a Ni layer having a thickness of 2 to 20μm is formed integrally with said metal jacket on an inner wall thereofby cladding; and said metal jacket is electrically connected to anegative electrode in said electrode assembly.
 7. The lithium secondarybattery in accordance with claim 6, wherein said magnesium-based alloyis produced by thixomolding.
 8. The lithium secondary battery inaccordance with claim 6, wherein said metal jacket is in a shape of abottomed can with an open top, having a bottom/side wall thickness ratioof 1.1 to 2.0; and said magnesium-based alloy is produced bythixomolding.
 9. A method of manufacturing a lithium secondary battery,comprising the steps of: (1) preparing a sheet of a magnesium-basedalloy containing lithium in an amount of 7 to 15% by weight, and atleast one element selected from the group consisting of Al, Zn, and Mnin a total amount of 0.3 to 5% by weight by thixomolding; (2) forming aNi layer integrally with said sheet on at least one face thereof bycladding; (3) forming a metal jacket in a shape of a bottomed can withan open top with said Ni layer formed on an inner wall thereof from saidsheet by a mechanical processing selected from drawing, combined drawingand ironing, and impact; and (4) placing an electrode assembly and anon-aqueous electrolyte into said metal jacket.
 10. A lithium secondarybattery comprising an electrode assembly and a non-aqueous electrolyte,both accommodated in a metal jacket, wherein said metal jacket is madeof a magnesium-based alloy containing lithium in an amount of 7 to 20%by weight; and an insulating layer is formed integrally with said metaljacket on an inner wall thereof.
 11. The lithium secondary battery inaccordance with claim 10, wherein said magnesium-based alloy containslithium in an amount of 7 to 15% by weight, and at least one elementselected from the group consisting of Al, Zn, Mn, Zr, Ca, Si, and rareearth elements in a total amount of 0.3 to 5% by weight.
 12. The lithiumsecondary battery in accordance with claim 10, wherein saidmagnesium-based alloy is a binary alloy containing lithium in an amountof 12 to 16% by weight.
 13. The lithium secondary battery in accordancewith claim 10, wherein said insulating layer comprises a metal oxide ora resin.
 14. A lithium secondary battery comprising an electrodeassembly and a non-aqueous electrolyte, both accommodated in a metaljacket, wherein said metal jacket is made of a magnesium-based alloycontaining lithium in an amount of 7 to 15% by weight, and at least oneelement selected from the group consisting of Al, Zn, and Mn in a totalamount of 0.3 to 5% by weight; and a resin layer having a thickness of 5μm or more is formed integrally with said metal jacket on an inner wallthereof.
 15. The lithium secondary battery in accordance with claim 14,wherein said magnesium-based alloy is produced by thixomolding.
 16. Thelithium secondary battery in accordance with claim 14, wherein saidmetal jacket is in a shape of a bottomed can with an open top, having abottom/side wall thickness ratio of 1.1 to 2.0; and said magnesium-basedalloy is produced by thixomolding.
 17. A method of manufacturing alithium secondary battery, comprising the steps of: (1) preparing asheet of a magnesium-based alloy containing lithium in an amount of 7 to15% by weight, and at least one element selected from the groupconsisting of Al, Zn, and Mn in a total amount of 0.3 to 5% by weight bythixomolding; (2) forming a resin layer integrally with said sheet on atleast one face thereof; (3) forming a metal jacket in a shape of abottomed can with an open top with said resin layer formed on an innerwall thereof from said sheet by a mechanical processing selected fromdrawing, combined drawing and ironing, and impact; and (4) placing anelectrode assembly and a non-aqueous electrolyte into said metal jacket.18. A method of manufacturing a lithium secondary battery, comprisingthe steps of: (1) preparing a sheet of a magnesium-based alloycontaining lithium in an amount of 7 to 15% by weight, and at least oneelement selected from the group consisting of Al, Zn, and Mn in a totalamount of 0.3 to 5% by weight by thixomolding; (2) forming a metaljacket in a shape of a bottomed can with an open top from said sheet bya mechanical processing selected from drawing, combined drawing andironing, and impact; (3) forming a resin layer integrally with saidmetal jacket on an inner wall thereof; and (4) placing an electrodeassembly and a non-aqueous electrolyte into said metal jacket.